User Guide: DuoSpray Ion Source for TripleTOF Systems Operator Guide

Principles of Operation—Ion Source

Figure A-1 Ion Evaporation

Item

1

2

3

4

Description

Droplet contains ions of both polarities with one polarity being predominant.

As the solvent evaporates, the electrical field increases and the ions move to the surface.

At some critical field value, ions are emitted from the droplets.

Nonvolatile residue remains as a dry particle.

If the droplet contains excess ions and enough solvent evaporates from the droplet, a critical field is reached at which ions are emitted from the surface. Eventually, all of the solvent will evaporate from the droplet, leaving a dry particle consisting of the nonvolatile components of the sample solution.

Because the solvation energies for most organic molecules are unknown, the sensitivities of any given organic ion to ion evaporation are difficult to predict. The importance of solvation energy is evident because surfactants that concentrate at the surface of a liquid can be detected very sensitively.

APCI Mode

The basis for past incompatibilities in linking liquid chromatography with mass spectrometry arose from difficulties converting relatively involatile molecules in solution in a liquid into a molecular gas without inducing excessive decomposition. The APCI probe process of gently nebulizing the sample into finely dispersed small droplets in a heated ceramic tube results in the rapid vaporization of the sample so that the sample molecules are not decomposed.

Figure A-2

shows the reaction flow of the APCI process for reactant positive ions (the proton hydrates,

H

3

O

+

[H

2

O] n

).

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Figure A-2 APCI Reaction Flow Diagram

Principles of Operation—Ion Source

The major primary ions N2

+

, O

2

+

, H

2

O

+

, and NO

+

are formed by the electron impact of corona created electrons on the major neutral components of air. Although NO

+

is normally not a major constituent of clean air, the concentration of this species in the source is enhanced due to neutral reactions initiated by the corona discharge.

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Principles of Operation—Ion Source

Samples that are introduced through the APCI probe are sprayed, with the aid of a nebulizer gas, into the heated ceramic tube. Within the tube, the finely dispersed droplets of sample and solvent undergo a rapid vaporization with minimal thermal decomposition. The gentle vaporization preserves the molecular identity of the sample.

The gaseous sample and solvent molecules pass into the ion source housing where the ionization by APCI is induced by a corona discharge needle connected to the end of the ceramic tube. The sample molecules are ionized by colliding with the reagent ions created by the ionization of mobile phase solvent molecules. As shown in

Figure

A-3

, the vaporized solvent molecules ionize to produce the reagent ions [X+H]+ in the positive mode and [X-H]– in the negative mode. It is these reagent ions that produce stable sample ions when they collide with the sample molecules.

Figure A-3 Atmospheric Pressure Chemical Ionization

Item

1

2

3

4

Description

Sample

Primary ions are created in the vicinity of the corona discharge needle

Ionization produces predominantly solvent ions

Reagent ions react with sample molecules forming clusters

5

6

Curtain plate

Interface x = solvent molecules; M=sample molecules

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